Semiconductor device

ABSTRACT

The semiconductor device of the present invention includes a first conductivity type semiconductor layer made of a wide bandgap semiconductor and a Schottky electrode formed to come into contact with a surface of the semiconductor layer, and has a threshold voltage V th  of 0.3 V to 0.7 V and a leakage current J r  of 1×10 −9  A/cm 2  to 1×10 −4  A/cm 2  in a rated voltage V R .

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 14/796,375, filed onJul. 10, 2015, and allowed on Oct. 7, 2016, which is a continuation ofU.S. application Ser. No. 14/235,784, filed on Feb. 12, 2014 (issued onAug. 18, 2015 as U.S. Pat. No. 9,111,852), which was a National Stageapplication of PCT/JP2012/069208, filed on Jul. 27, 2012, and claims thebenefit of priority of Japanese Patent Application No. 2011-165660,filed on Jul. 28, 2011. The disclosures of these prior U.S. and foreignapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device that includes aSchottky barrier diode made of a wide bandgap semiconductor.

BACKGROUND ART

Heretofore, attention has been paid to a semiconductor device(semiconductor power device) for use chiefly in a system in variouspower electronics fields, such as a motor control system or a powerconversion system.

For example, FIG. 1 of Patent Literature 1 discloses a Schottky barrierdiode in which SiC is employed. This Schottky barrier diode is composedof an n type 4H—SiC bulk substrate, an n type epitaxial layer that hasgrown on the bulk substrate, an oxide film that is formed on a surfaceof the epitaxial layer and that partially exposes the surface of theepitaxial layer, and a Schottky electrode that is formed in an openingof the oxide film and that makes a Schottky junction with the epitaxiallayer.

FIG. 8 of Patent Literature 1 discloses a vertical MIS field-effecttransistor in which SiC is employed. This vertical MIS field-effecttransistor is composed of an n type 4H—SiC bulk substrate, an n typeepitaxial layer that has grown on the bulk substrate, an n type impurityregion (source region) that is formed on a surface layer part of theepitaxial layer, a p type well region that is formed adjacently to bothsides of the n type impurity region, a gate oxide film that is formed ona surface of the epitaxial layer, and a gate electrode that faces the ptype well region through the gate oxide film.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Publication No. 2005-79339

PTL 2: Japanese Unexamined Patent Publication No. 2011-9797

SUMMARY OF INVENTION Solution to Problem

The semiconductor device of the present invention includes a firstconductivity type semiconductor layer made of a wide bandgapsemiconductor and a Schottky electrode formed to come into contact witha surface of the semiconductor layer, in which a threshold voltageV_(th) is 0.3 V to 0.7 V, and a leakage current J_(r) in a rated voltageV_(R) is 1×10⁻⁹ A/cm² to 1×10⁻⁴ A/cm².

According to this arrangement, a threshold voltage Vth is 0.3 V to 0.7V, and a leakage current J_(r) in a rated voltage V_(R) is 1×10⁻⁹ A/cm²to 1×10⁻⁴ A/cm², and therefore a current-carrying loss can be reduced tobe equal to or to be smaller than that of an Si-pn diode while aswitching loss can be smaller than the Si-pn diode. As a result, it isbuilt in a power module for use in, for example, an inverter circuitthat forms a driving circuit to drive an electric motor used as a powersource for electric vehicles (including hybrid automobiles), trains,industrial robots, etc., and hence it is possible to achieve a powermodule that is high in withstanding pressure and that is low in loss.

Preferably, when a breakdown voltage V_(B) of the semiconductor deviceis 700 V or more, the rated voltage V_(R) of the semiconductor device is50 to 90% of the breakdown voltage V_(B) that is 700 V or more.

Additionally, preferably, on-resistance R_(on)·A of the semiconductordevice is 0.3 mΩ·cm² to 3 mΩ·cm².

Preferably, in order to set the threshold voltage V_(th) of thesemiconductor device at 0.3 V to 0.7 V and in order to set the leakagecurrent J_(r) in the rated voltage V_(R) at 1×10⁻⁹ A/cm² to 1×10⁻⁴A/cm², for example, a trench having a side wall and a bottom wall isformed on the surface side of the semiconductor layer, and an edge partof the bottom wall of the trench has a curvature radius R that satisfiesthe following formula (1):

0.01L<R<10L  (1)

(In formula (1), L designates a linear distance between edge partsfacing each other along a width direction of the trench.)

The wide bandgap semiconductor has a breakdown voltage V_(B) extremelyhigher than silicon, and a semiconductor device using such a widebandgap semiconductor can fulfill high pressure resistance. This resultsfrom the fact that the wide bandgap semiconductor is extremely higher ininsulation breakdown electric field strength than silicon. Therefore, itis possible to design a device having a comparatively high rated voltageV_(R) by use of a Schottky barrier diode structure.

Therefore, when a high reverse voltage is applied to such a Schottkybarrier diode, a high electric field is applied to the wide bandgapsemiconductor even if the diode does not break down although acomparatively high voltage can be treated in the Schottky barrier diode.Therefore, if the height (barrier height) of a Schottky barrier betweenthe Schottky electrode and the wide bandgap semiconductor is lowered inorder to reduce the threshold voltage V_(th) of the Schottky barrierdiode, a leakage current J_(r) (reverse leakage current) flowing beyondthe Schottky barrier during application of a reverse voltage willincrease because the electric field strength of the wide bandgapsemiconductor and that of the Schottky interface are great.

From the viewpoint of preventing an increase in reverse leakage currentJ_(r), in a Schottky barrier diode having a wide bandgap semiconductor,a high reverse voltage is required not to be applied, and the barrierheight is required to be increased to some extent. As a result,disadvantageously, the pressure resistance of the wide bandgapsemiconductor that makes it possible to prevent a breakdown cannot beefficiently utilized even if a high reverse voltage is applied.

Here, let it be considered the distribution of electric field strengthwhen a reverse voltage is applied. First, when a reverse voltage isapplied to a semiconductor layer (e.g., n type) that is made of a widebandgap semiconductor and that is not provided with a trench, theelectric field strength usually becomes higher in proportion to anapproach to the surface from the reverse surface of the semiconductorlayer, and reaches the maximum at the surface of the semiconductorlayer.

Therefore, in a Schottky barrier diode in which a Schottky electrode isallowed to make a Schottky junction with the surface of a semiconductorlayer having such a structure and in which the height (barrier height)of a Schottky barrier between the Schottky electrode and thesemiconductor layer is lowered, it is difficult to reduce a reverseleakage current J_(r) flowing beyond the Schottky barrier because theelectric field strength at the surface of the semiconductor layer ishigh when a reverse voltage closer to a breakdown voltage V_(B) isapplied.

Therefore, although it is conceivable that a trench is formed at thesemiconductor layer and that a part (generation source of a leakagecurrent) of the semiconductor layer on which an electric field isconcentrated is shifted to a bottom part of the trench, the electricfield will concentrate on an edge part of the bottom wall of the trenchif so, and therefore a problem arises in which sufficient withstandingpressure cannot be obtained if the edge part has a sharp shape.

Therefore, according to the present invention, the electric field thatconcentrates on the edge part of the bottom wall of the trench can bemoderated to improve withstanding pressure by setting the curvatureradius R of the edge part of the bottom wall of the trench so as tosatisfy the relation 0.01L<R<10L. Of course, the electric field strengthin the surface of the semiconductor layer can be weakened because atrench is formed on the surface side of the semiconductor layer. As aresult, the reverse leakage current J_(r) can be set at 1×10⁻⁹ A/cm² to1×10⁻⁴ A/cm² even if a barrier height between the Schottky electrode andthe semiconductor layer contiguous to the surface of the semiconductorlayer is lowered and even if the reverse voltage closer to a breakdownvoltage is applied. As a result, the threshold voltage V_(th) can bereduced to be 0.3 V to 0.7 V by lowering the barrier height while thereverse leakage current J_(r) can be reduced.

Preferably, in the semiconductor device of the present invention, thesemiconductor layer includes a second conductivity typeelectric-field-moderating portion that is selectively formed at thebottom wall of the trench and at the edge part of the bottom wall.

In other words, preferably, in the present invention, anelectric-field-moderating portion of a second conductivity type (e.g., ptype) is additionally formed at the bottom wall of the trench and at theedge part of the bottom wall. This makes it possible to further reducethe reverse leakage current J_(r) as the whole of the semiconductordevice. In other words, the reverse leakage current J_(r) can be madeeven smaller even if a reverse voltage closer to a breakdown voltageV_(B) is applied, and therefore the pressure resistance of a widebandgap semiconductor can be satisfactorily utilized.

In this case, more preferably, the electric-field-moderating portion isformed to straddle between the edge part of the bottom wall of thetrench and the side wall of the trench, and, particularly preferably,the electric-field-moderating portion is formed to lead to an openingend of the trench along the side wall of the trench.

In the present invention, the Schottky electrode is a concept thatincludes both a metal electrode that makes a Schottky barrier with asemiconductor layer and a semiconductor electrode that is made of adissimilar semiconductor having a bandgap differing from the bandgap ofthe semiconductor layer and that makes a heterojunction with thesemiconductor layer (junction that forms a potential barrier with thesemiconductor layer by using a bandgap difference). Hereinafter, in thisdescription division, the Schottky junction and the heterojunction willbe referred to generically as “Schottky junction,” and the Schottkybarrier and the potential barrier (heterobarrier) formed by theheterojunction will be referred to generically as “Schottky barrier,”and the metal electrode and the semiconductor electrode will be referredto generically as “Schottky electrode.”

Preferably, the trench includes a taper trench that has the bottom wallhaving a planar shape and the side wall inclined at an angle exceeding90° with respect to the bottom wall having a planar shape.

If it is a taper trench, the withstanding pressure of the semiconductordevice can be made even higher than when the side wall is erectedrectangularly at 90° with respect to the bottom wall.

Additionally, in the taper trench, not only the bottom wall but also apart of or all of the side wall faces the open end of the trench.Therefore, for example, when a second conductivity type impurity isimplanted to the semiconductor layer through the trench, an impuritythat has entered the inside of the trench from the open end of thetrench can be allowed to reliably impinge on the side wall of thetrench. As a result, the aforementioned electric-field-moderatingportion can be formed easily.

The taper trench is a concept that includes both a trench in which allof the side wall is inclined at an angle exceeding 90° with respect tothe bottom wall and a trench in which a part of the side wall (e.g.,part that forms the edge part of the trench) is inclined at an angleexceeding 90° with respect to the bottom wall.

Preferably, in the semiconductor device of the present invention, theSchottky electrode is formed so as to be embedded in the trench, and theelectric-field-moderating portion has a contact portion that makes anohmic contact with the Schottky electrode embedded in the trench at apart forming the bottom wall of the trench.

According to this arrangement, the Schottky electrode can be allowed tomake an ohmic contact with the pn diode having a pn junction between theelectric-field-moderating portion (second conductivity type) and thesemiconductor layer (first conductivity type). This pn diode is disposedin parallel with the Schottky barrier diode (heterodiode) having aSchottky junction between the Schottky electrode and the semiconductorlayer. This makes it possible to allow a part of a surge current to flowto a built-in pn diode even if this surge current flows to thesemiconductor device. As a result, the surge current flowing through theSchottky barrier diode can be reduced, and therefore the Schottkybarrier diode can be prevented from being thermally broken down by thesurge current.

Preferably, in the semiconductor device of the present invention, if thesemiconductor layer has a first part of a first conductivity type towhich a first electric field is applied when a reverse voltage isapplied and a second part of the first conductivity type to which asecond electric field relatively higher than the first electric field isapplied, the Schottky electrode includes a first electrode that forms afirst Schottky barrier with the first part and a second electrode thatforms a second Schottky barrier, which is relatively higher than thefirst Schottky barrier, with the second part.

In the present invention, there is a case in which a part having arelatively high electric field strength and a part having a relativelylow electric field strength are present as shown in a relationshipbetween the first part and the second part of the semiconductor layer.

Therefore, if the Schottky electrode is properly selected in accordancewith the electric field distribution of the semiconductor layer when areverse voltage is applied as described above, a leakage current can berestrained by the comparatively high second Schottky barrier in thesecond part to which the relatively high second electric field isapplied when a reverse voltage is applied. On the other hand, in thefirst part to which the relatively low first electric field is applied,the fear that the reverse leakage current will flow beyond the Schottkybarrier is slight even if the height of the Schottky barrier is lowered,and therefore an electric current can be allowed to preferentially flowat a low voltage during application of a forward voltage by setting thecomparatively low first Schottky barrier. Therefore, according to thisarrangement, the reverse leakage current J_(r) and the threshold voltageV_(th) can be efficiently reduced.

For example, when the electric-field-moderating portion is formed tolead to the opening end of the trench, the first part of thesemiconductor layer is formed at a peripheral edge of the opening end ofthe trench in a surface layer part of the semiconductor layer, whereasthe second part of the semiconductor layer is formed at a part adjoiningthe peripheral edge in the surface layer part of the semiconductorlayer.

Preferably, in the semiconductor device of the present invention, whenthe semiconductor layer includes a base drift layer that has a firstimpurity concentration and a low-resistance drift layer that is formedon the base drift layer and that has a second impurity concentrationrelatively higher than the first impurity concentration, the trench isformed so that a deepest part thereof reaches the low-resistance driftlayer, and a part of the semiconductor layer is partitioned as a unitcell.

In the unit cell partitioned by the trench, an area (current path) inwhich an electric current can be allowed to flow is restricted, andtherefore there is a fear that the resistance value of the unit cellwill rise if the impurity concentration of a part that forms the unitcell in the semiconductor layer is low. Therefore, as described above,all of or part of the unit cell can be formed with the low-resistancedrift layer by forming the trench so that the deepest part reaches thelow-resistance drift layer. Therefore, a rise in the resistance valuecan be restrained by the low-resistance drift layer having thecomparatively high second impurity concentration even if the currentpath is narrowed in a part in which the low-resistance drift layer isformed. As a result, the unit cell can be made low in resistance.

The first impurity concentration of the base drift layer may becomelower in proportion to an approach to the surface from a reverse surfaceof the semiconductor layer. Additionally, the second impurityconcentration of the low-resistance drift layer may be constant inproportion to an approach to the surface from the reverse surface of thesemiconductor layer, or may become lower in proportion to an approach tothe surface from the reverse surface of the semiconductor layer.

Preferably, the semiconductor layer further includes an obverse-surfacedrift layer that is formed on the low-resistance drift layer and thathas a third impurity concentration relatively lower than the secondimpurity concentration.

This arrangement makes it possible to reduce the impurity concentrationof the surface layer part of the semiconductor layer, and thereforemakes it possible to reduce the electric field strength applied to thesurface layer part of the semiconductor layer when a reverse voltage isapplied. As a result, the reverse leakage current J_(r) can be made evensmaller.

The semiconductor layer may further include a substrate and a bufferlayer that is formed on the substrate and that has a fourth impurityconcentration relatively higher than the first impurity concentration.

Additionally, the trench may include a stripe trench formed in a stripemanner, and may include a lattice trench formed in a grid-like manner.

Additionally, the chip size of the semiconductor device may be 0.5 mm/□to 20 mm/□.

The wide bandgap semiconductor (whose bandgap is 2 eV or more) is asemiconductor in which an insulation breakdown electric field is greaterthan 1 MV/cm, and, more specifically, the wide bandgap semiconductor ismade of SiC (e.g., 4H—SiC whose insulation breakdown electric field isabout 2.8 MV/cm and whose bandgap width is about 3.26 eV), GaN (whoseinsulation breakdown electric field is about 3 MV/cm and whose bandgapwidth is about 3.42 eV), or diamond (whose insulation breakdown electricfield is about 8 MV/cm and whose bandgap width is about 5.47 eV).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A and FIG. 1B are schematic plan views of a Schottky barrier diodeaccording to an embodiment of the present invention, FIG. 1A being anoverall view, FIG. 1B being an enlarged view of a main part.

FIG. 2 is a sectional view of the Schottky barrier diode shown in FIG.1A and FIG. 1B, showing a cutting plane in cutting-plane line A-A ofFIG. 1B.

FIG. 3 is an enlarged view of a trench of FIG. 2.

FIG. 4 is a distribution view (simulation data) of electric fieldstrength when a reverse voltage is applied, showing a case in which atrench structure is absent.

FIG. 5 is a distribution view (simulation data) of electric fieldstrength when a reverse voltage is applied, showing a case in which arectangular trench structure is present.

FIG. 6 is a distribution view (simulation data) of electric fieldstrength when a reverse voltage is applied, showing a case in which aU-shaped trench structure is present.

FIG. 7 is a distribution view (simulation data) of electric fieldstrength when a reverse voltage is applied, showing a case in which atrapezoidal trench structure is present.

FIG. 8 is a distribution view (simulation data) of electric fieldstrength when a reverse voltage is applied, showing a case in which atrapezoid trench structure+a bottom-wall p type layer are present.

FIG. 9 is a distribution view (simulation data) of electric fieldstrength when a reverse voltage is applied, showing a case in which atrapezoid trench structure+a side-wall p type layer is present.

FIG. 10 is a schematic sectional view of a Schottky barrier diode thathas a JBS structure.

FIG. 11 is a schematic sectional view of a Schottky barrier diode thathas a pseudo-JBS structure.

FIG. 12 is a schematic sectional view of a Schottky barrier diode thathas a planar structure.

FIG. 13 is a graph showing a relationship between a threshold voltageV_(th) and a leakage current J_(r) of each Schottky barrier diode.

FIG. 14 is a graph showing a relationship between a threshold voltageV_(th) and on-resistance R_(on) of each Schottky barrier diode.

FIG. 15 is a graph showing a relationship between a threshold voltageV_(th) and a breakdown voltage V_(B) of each Schottky barrier diode.

FIG. 16 is a graph showing a current-voltage (I-V) curve of a built-inpn junction portion.

FIG. 17 is an enlarged view of a main part of the distribution view ofthe electric field strength shown in FIG. 9, in which a part near thetrench of the Schottky barrier diode is enlarged.

FIG. 18 is a graph showing the electric field strength distribution in asurface of a unit cell of the Schottky barrier diode shown in FIG. 17.

FIG. 19 is a view to describe the impurity concentration of an SiCsubstrate and the impurity concentration of an SiC epitaxial layer.

FIG. 20A is a view showing a method for forming the trench and the ptype layer shown in FIG. 2.

FIG. 20B is a view showing a step following FIG. 20A.

FIG. 20C is a view showing a step following FIG. 20B.

FIG. 20D is a view showing a step following FIG. 20C.

FIG. 21 is a schematic view that represents a unit cell having a 4H-SiCcrystal structure.

FIGS. 22A, 22B, 22C, 22D, 22E, and 22F are views showing modificationsof the cross-sectional shape of a trench, FIG. 22A being a firstmodification, FIG. 22B being a second modification, FIG. 22C being athird modification, FIG. 22D being a fourth modification, FIG. 22E beinga fifth modification, FIG. 22F being a sixth modification.

FIG. 23A is a view showing a method for forming the trench and the ptype layer shown in FIG. 22B.

FIG. 23B is a view showing a step following FIG. 23A.

FIG. 23C is a view showing a step following FIG. 23B.

FIG. 23D is a view showing a step following FIG. 23C.

FIG. 24A is a view showing a method for forming the trench and the ptype layer shown in FIG. 22D.

FIG. 24B is a view showing a step following FIG. 24A.

FIG. 24C is a view showing a step following FIG. 24B.

FIG. 24D is a view showing a step following FIG. 24C.

FIG. 24E is a view showing a step following FIG. 24D.

FIG. 24F is a view showing a step following FIG. 24E.

FIG. 24G is a view showing a step following FIG. 24F.

FIG. 25 is a view showing a modification of the planar shape of atrench.

FIG. 26 is a view showing an example (first mode) in which an insulatingfilm is formed on a surface of a trench.

FIG. 27 is a view showing an example (second mode) in which aninsulating film is formed on a surface of a trench.

FIG. 28 is a view showing an example (third mode) in which an insulatingfilm is formed on a surface of a trench.

FIG. 29 is a view showing an example (fourth mode) in which aninsulating film is formed on a surface of a trench.

FIG. 30 is a view showing an example (fifth mode) in which an insulatingfilm is formed on a surface of a trench.

FIG. 31 is a view showing an example (sixth mode) in which an insulatingfilm is formed on a surface of a trench.

FIG. 32 is a view showing an example (seventh mode) in which aninsulating film is formed on a surface of a trench.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be hereinafter described indetail with reference to the accompanying drawings.

<Entire Structure of Schottky Barrier Diode>

FIG. 1A and FIG. 1B are schematic plan views of a Schottky barrier diodeaccording to an embodiment of the present invention, and FIG. 1A is anoverall view, and FIG. 1B is an enlarged view of a main part. FIG. 2 isa sectional view of the Schottky barrier diode shown in FIG. 1A and FIG.1B, and shows a cutting plane in cutting-plane line A-A of FIG. 1B. FIG.3 is an enlarged view of a trench of FIG. 2.

The Schottky barrier diode 1 serving as a semiconductor device is aSchottky barrier diode in which 4H—SiC (a wide bandgap semiconductorwhose insulation breakdown electric field is about 2.8 MV/cm and whosebandgap width is about 3.26 eV) is employed, and, for example, is shapedlike a chip having a square shape when viewed planarly. In thechip-shaped Schottky barrier diode 1, the length in each of the up,down, right, and left directions in the sheet of FIG. 1A is 0.5 mm to 20mm. In other words, the chip size of the Schottky barrier diode 1 is,for example, 0.5 mm/□ to 20 mm/□.

The Schottky barrier diode 1 includes an n⁺ type SiC substrate 2. Thethickness of the SiC substrate 2 is, for example, 50 μm to 600 μm. Forexample, N (nitrogen), P (phosphorus), As (arsenic), etc., can be usedas n type impurities.

A cathode electrode 4 serving as an ohmic electrode is formed on areverse surface 3 of the SiC substrate 2 so as to cover its whole area.The cathode electrode 4 is made of a metal (e.g., Ti/Ni/Ag) that comesinto ohmic contact with the n type SiC.

An n type SiC epitaxial layer 6 serving as a semiconductor layer isformed on a surface 5 of the SiC substrate 2.

The SiC epitaxial layer 6 has a laminated structure in which a bufferlayer 7 and a drift layer having a three-layer structure consisting of abase drift layer 8, a low-resistance drift layer 9, and anobverse-surface drift layer 10 are stacked together in this order fromthe surface 5 of the SiC substrate 2. The buffer layer 7 forms a reversesurface 11 of the SiC epitaxial layer 6, and is in contact with thesurface 5 of the SiC substrate 2. On the other hand, the obverse-surfacedrift layer 10 forms a surface 12 of the SiC epitaxial layer 6.

The total thickness T of the SiC epitaxial layer 6 is, for example, 3 μmto 100 μm. The thickness t₁ of the buffer layer 7 is, for example, 0.1μm to 1 μm. The thickness t₂ of the base drift layer 8 is, for example,2 μm to 100 μm. The thickness t₃ of the low-resistance drift layer 9 is,for example, 1 μm to 3 μm. The thickness t₄ of the obverse-surface driftlayer 10 is, for example, 0.2 μm to 0.5 μm.

A field insulating film 16 is formed on the surface 12 of the SiCepitaxial layer 6. The field insulating film 16 has an opening 14 thatexposes a part of the SiC epitaxial layer 6 as an active region 13(whose active size is, for example, 0.1 mm² to 400 mm²) and covers afield region 15 surrounding the active region 13. The field insulatingfilm 16 is made of, for example, SiO₂ (silicon oxide). The thickness ofthe field insulating film 16 is, for example, 0.5 μm to 3 μm.

A stripe trench that penetrates the obverse-surface drift layer 10 fromthe surface 12 of the SiC epitaxial layer 6 and that has its deepestpart reaching a halfway part of the low-resistance drift layer 9 isformed on the side of the surface 12 in the active region 13. The stripetrench is formed such that a plurality of trapezoid trenches 17(trenches each of which has a reverse-trapezoidal shape in across-sectional view when it is cut along a width directionperpendicular to its longitudinal direction) extending linearly in adirection in which a couple of opposite sides of the Schottky barrierdiode 1 face each other are arranged parallel with each other atintervals. The center-to-center distance (pitch P) between the centersof adjoining trapezoid trenches 17 is, for example, 2 μm to 20 μm.

As a result, unit cells 18 (line cells) each of which is partitioned bybeing sandwiched between the adjoining trapezoid trenches 17 are formedin a stripe manner at the SiC epitaxial layer 6. In each unit cell 18, abase part that occupies most of its area is formed by the low-resistancedrift layer 9, and a surface layer part on the side of the surface 12with respect to the base part is formed by the obverse-surface driftlayer 10.

Each trapezoid trench 17 is partitioned by a bottom wall 20 that forms abottom surface 19 parallel to the surface 12 of the SiC epitaxial layer6 and by a side wall 22 forming a side surface 21 inclined at angle θ₁(e.g., 95° to 150°) with respect to the bottom surface 19 from an edgepart 24 of both ends in the width direction of the bottom wall 20 towardthe surface 12 of the SiC epitaxial layer 6. The depth of eachtrapezoidal trench 17 (i.e., distance from the surface 12 of the SiCepitaxial layer 6 to the bottom surface 19 of the trapezoidal trench 17)is, for example, 3000 Å to 15000 Å. The width W (width of the deepestpart) perpendicular to the longitudinal direction of each trapezoidtrench 17 is 0.3 μm to 10 μm.

As shown in FIG. 3, the edge part 24 of the bottom wall 20 of eachtrapezoidal trench 17 is formed to have a shape curved outwardly fromthe trapezoidal trench 17, and a bottom part of each trapezoidal trench17 is formed to have the shape of the letter U when viewedcross-sectionally. The curvature radius R of the inner surface (curvedplane) of the edge part 24 shaped in this way satisfies the followingformula (1).

0.01L<R<10L  (1)

In Formula (1), L designates a linear distance between the edge parts 24facing each other along the width direction of the trench 17 (nospecific limitations are imposed on the unit if it is a unit of lengthsuch as μm, nm, or m). More specifically, it is the width of the bottomsurface 19 parallel to the surface 12 of the SiC epitaxial layer 6, andis a value obtained by subtracting the width of the edge part 24 fromthe width W of the trench 17.

Preferably, the curvature radius R of the edge part 24 satisfies thefollowing formula (2):

0.02L<R<1L  (2)

The curvature radius R can be found, for example, by photographing thecross section of the trapezoidal trench 17 with a SEM (Scanning ElectronMicroscope) and by measuring the curvature of the edge part 24 of aresulting SEM image.

A p type layer 23 serving as an electric-field-moderating portion isformed along an inner surface of the trapezoidal trench 17 so as to beexposed to the inner surface at the bottom wall 20 and the side wall 22of the trapezoidal trench 17. The p type layer 23 is formed from thebottom wall 20 of the trapezoidal trench 17 to an opening end of thetrapezoidal trench 17 via the edge part 24. The p type layer 23 forms apn junction portion between the n type SiC epitaxial layer 6 and the ptype layer 23. As a result, a pn diode 25 composed of the p type layer23 and the n type SiC epitaxial layer 6 (low-resistance drift layer 9)is built in the Schottky barrier diode 1.

As shown in FIG. 3, in the thickness of the p type layer 23 (i.e., depthfrom the inner surface of the trapezoidal trench 17), a first thicknesst₅ from the bottom surface 19 of the trapezoidal trench 17 measured inthe depth direction of the trapezoidal trench 17 (i.e., directionperpendicular to the surface 12 of the SiC epitaxial layer 6) is greaterthan a second thickness t₆ from the side surface 21 of the trapezoidaltrench 17 measured in the width direction of the trapezoidal trench 17(i.e., direction parallel to the surface 12 of the SiC epitaxial layer6). More specifically, the first thickness t₅ is, for example, 0.3 μm to0.7 μm, and the second thickness t₆ is, for example, 0.1 μm to 0.5 μm.

The p type layer 23 has a p⁺ type contact portion 26 into whichimpurities have been implanted at a higher concentration than otherparts of the p type layer 23 at a part of the bottom wall 20 of thetrapezoidal trench 17. For example, the impurity concentration of thecontact portion 26 is 1×10²⁰ to 1×10²¹ cm⁻³, and the impurityconcentration of other parts of the electric-field-moderating portionexcluding the contact portion 26 is 1×10¹⁷ to 5×10¹⁸ cm⁻³.

The contact portion 26 is formed linearly along the longitudinaldirection of the trapezoidal trench 17, and has a depth (e.g., 0.05 μmto 0.2 μm) from the bottom surface 19 of the trapezoidal trench 17 to ahalfway point in the depth direction of the p type layer 23.

An anode electrode 27 serving as a Schottky electrode is formed on thefield insulating film 16.

The anode electrode 27 includes a first electrode 28 formed at a top ofeach unit cell 18 and a second electrode 29 that straddles between theadjoining trapezoidal trenches 17 and that is formed so as to cover thefirst electrode 28 at the top of the unit cell 18 sandwiched betweenthose trapezoidal trenches 17.

The first electrode 28 is formed linearly along the longitudinaldirection of the trapezoidal trench 17 in a central part 31 sandwichedbetween peripheral edges 30 of opening ends of the adjoining trapezoidaltrenches 17 at the top of each unit cell 18.

The second electrode 29 is formed so as to cover the whole of the activeregion 13, and is embedded in each trapezoidal trench 17. Additionally,the second electrode 29 projects in a flange-like manner outwardly fromthe opening 14 so as to cover the peripheral edge of the opening 14 inthe field insulating film 16 from above. In other words, the peripheraledge of the field insulating film 16 is sandwiched between the upper andlower sides over the entire perimeter by means of the SiC epitaxiallayer 6 (obverse-surface drift layer 10) and the second electrode 29.Therefore, the outer peripheral area of Schottky junction in the SiCepitaxial layer 6 (i.e., inner edge of the field region 15) is coveredwith the peripheral edge of the field insulating film 16 made of SiC.

An annular trench 32 that penetrates the obverse-surface drift layer 10from the surface 12 of the SiC epitaxial layer 6 and that has itsdeepest part reaching a halfway part of the low-resistance drift layer 9is formed on the side of the surface 12 of the SiC epitaxial layer 6 inthe field region 15. The annular trench 32 is formed such that aplurality of trenches surrounding the active region 13 are arrangedparallel with each other at intervals. The interval between the annulartrenches 32 adjoining each other is set to become greater in proportionto an approach to a far side from a near side with respect to the activeregion 13. As a result, the width of a part sandwiched between theannular trenches 32 adjoining each other becomes greater in proportionto an approach to the far side from the near side with respect to theactive region 13.

A p type layer 49 is formed on a bottom wall 50 and a side wall 51 ofthe annular trench 32 along an inner surface of the annular trench 32 soas to be exposed to this inner surface. The p type layer 49 is formedfrom the bottom wall 50 of the annular trench 32 to an opening end ofthe annular trench 32 via an edge part 52 at both ends in the widthdirection of the bottom wall 50 in the same way as the p type layer 23.

This p type layer 49 is formed at the same step as the p type layer 23,and has the same impurity concentration (e.g., 1×10¹⁷ to 5×10¹⁸ cm⁻³)and the same thickness as the p type layer 23.

A surface protection film 33 made of, for example, silicon nitride (SiN)is formed on the topmost surface of the Schottky barrier diode 1. Anopening 34 by which the anode electrode 27 (second electrode 29) isexposed is formed at a central part of the surface protection film 33. Abonding wire etc., are bonded to the second electrode 29 through thisopening 34.

In the Schottky barrier diode 1, a forward bias state is reached inwhich a positive voltage is applied to the anode electrode 27 and inwhich a negative voltage is applied to the cathode electrode 4, and, asa result, electrons (carriers) move from the cathode electrode 4 to theanode electrode 27 through the active region 13 of the SiC epitaxiallayer 6, and an electric current flows.

In the Schottky barrier diode 1, its threshold voltage V_(th) is 0.3V to0.7V, and its leakage current J_(r) in the rated voltage V_(R) is 1×10⁻⁹A/cm² to 1×10⁻⁴ A/cm².

The threshold voltage V_(th) can be found, for example, from a voltagevalue indicated by an intersection between an extension line of a linearpart of an I-V curve and the X axis in a graph (X axis: voltage, Y axis:electric current) showing I-V characteristics of the Schottky barrierdiode 1.

The rated voltage V_(R) is, for example, 50 to 90% of a breakdownvoltage V_(B), and the breakdown voltage V_(B) can be found by thefollowing formula (3). In the present embodiment, the breakdown voltageV_(B) is 700 V or more (specifically, 700 V to 3000 V).

$\begin{matrix}\left\lbrack {{Numerical}\mspace{14mu} {Formula}\mspace{14mu} 1} \right\rbrack & \; \\{V_{BR} = \sqrt{\frac{W\; ɛ\; E^{3}}{4{qN}}}} & (3)\end{matrix}$

(In Formula (3), W designates the thickness of the SiC epitaxial layer6, E designates the insulation breakdown electric field strength of theSiC epitaxial layer 6, q designates elementary charge, and N designatesthe impurity concentration of the SiC epitaxial layer 6.)

The on-resistance R_(on)·A of the Schottky barrier diode 1 is 0.3 mΩ·cm²to 3 mΩ·cm².

The fact that the Schottky barrier diode 1 of the present embodiment hasthe threshold voltage V_(th) and the leakage current J_(r) fallingwithin the aforementioned range can be proven by the following item<Introduction Effect of Trench Structure>.

<Introduction Effect of Trench Structure>

Referring to FIG. 4 to FIG. 15, a description will be given of areduction effect of the reverse leakage current J_(r) and the thresholdvoltage V_(th) brought about by forming the trapezoidal trench 17 andthe p type layer 23 in the SiC epitaxial layer 6. It should be notedthat the trench of FIG. 5 is a rectangular trench 17′, and the trench ofFIG. 6 is a U-shaped trench 17″.

FIG. 4 to FIG. 9 are distribution views (simulation data) of electricfield strength when a reverse voltage is applied, FIG. 4 showing a casein which a trench structure is absent, FIG. 5 showing a case in which arectangular trench structure is present, FIG. 6 showing a case in whicha U-shaped trench structure (θ₁=90°, R=0.125 L or 1/(1×10⁷)(m)) ispresent, FIG. 7 showing a case in which a trapezoidal trench structure(θ₁=115°>90°, R=0.125 L or 1/(1×10⁷)(m)) is present, FIG. 8 showing acase in which a trapezoidal trench structure (θ₁=115°>90°, R=0.125 L or1/(1×10⁷)(m))+a bottom-wall p type layer are present, FIG. 9 showing acase in which a trapezoidal trench structure (θ₁=115°>90°, R=0.125 L or1/(1×10⁷)(m))+a side-wall p type layer are present. In FIG. 4 to FIG. 9,the same reference sign as in FIGS. 1A, 1B, 2, and 3 are given to acomponent equivalent to each component shown in FIGS. 1A, 1B, 2, and 3.

First, the structures of FIG. 4 to FIG. 9 were designed as follows.

n⁺ type SiC substrate 2: 1×10¹⁹ cm⁻³ in concentration, 1 μm in thickness

n⁻ type SiC epitaxial layer 6: 1×10¹⁶ cm⁻³ in concentration, 5 μm inthickness

Trenches 17, 17′, and 17″: 1.05 μm in depth

Curvature radius R of edge part 24 of bottom wall 20:

p type layer 23: 1×10¹⁸ cm⁻³ in concentration

Thereafter, the electric field strength distribution in the SiCepitaxial layer 6 was simulated when a reverse voltage (600 V) wasapplied to an anode-to-cathode interval of the Schottky barrier diode 1having each of the structures of FIG. 4 to FIG. 9. A TCAD (product name)made by Synopsys, Inc. was used as a simulator.

As shown in FIG. 4, it has been recognized that no trench structurehaving a shape is formed, and, in the Schottky barrier diode in whichthe surface 12 of the SiC epitaxial layer 6 is flat, the electric fieldstrength becomes greater in proportion to an approach to the surface 12from the reverse surface 11 of the SiC epitaxial layer 6, and reachesthe maximum (about 1.5×10⁶ V/cm) at the surface 12 of the SiC epitaxiallayer 6.

Additionally, as shown in FIG. 5, it has been recognized that, in theSchottky barrier diode in which a rectangular trench structure having asharply shaped edge part 24 is formed, the electric field strength at apart (unit cell 18) sandwiched between the rectangular trenches 17′adjoining each other is weakened by forming the structure of therectangular trench 17′ (the electric field strength at the central part31 of the unit cell 18 is about 9×10⁵ V/cm), and an intense electricfield of about 1.5×10⁶ V/cm is concentrated on the edge part 24 of thebottom wall 20 of the rectangular trench 17′.

On the other hand, as shown in FIG. 6 and FIG. 7, it has been recognizedthat, in the Schottky barrier diode in which the structures of theU-shaped trench 17″ and the trapezoidal trench 17 are formed and inwhich the P type layer 23 is not formed on the inner walls of thesetrenches 17 and 17″, the electric field strength of a part (unit cell18) sandwiched between the trapezoidal trenches 17 adjoining each otheris weakened by forming the structures of the trenches 17 and 17″, and apart in which the electric field strength reaches the maximum is shiftedto the whole of the bottom wall 20 of the trapezoidal trench 17. Morespecifically, the electric field strength of the central part 31 of theunit cell 18 was weakened to about 9×10⁵ V/cm, and the electric fieldstrength of the peripheral edge 30 of the unit cell 18 was weakened toabout 3×10⁵ V/cm, and the electric field strength of the whole of thebottom wall 20 of the trapezoidal trench 17 reached the maximum showingabout 1.5×10⁶ V/cm. In other words, it has been recognized that thelocal concentration of an electric field on the edge part 24 can bemoderated.

Therefore, even if a barrier height between the SiC epitaxial layer 6and the anode electrode 27 (Schottky electrode) contiguous to thesurface 12 (surface of the unit cell 18) of the SiC epitaxial layer 6 islowered and a reverse voltage closer to a breakdown voltage is applied,the electric field strength of a part in which this barrier height isformed is weak, and therefore it has been recognized that the absoluteamount of reverse leakage current J_(r) that exceeds this barrier heightcan be reduced. As a result, it has been recognized that the thresholdvoltage V_(th) can be reduced by lowering the barrier height while thereverse leakage current J_(r) can be reduced.

On the other hand, a part (generation source of a leakage current) onwhich an electric field is concentrated in the SiC epitaxial layer 6 isshifted to the bottom part of trenches 17 and 17″ by forming theU-shaped trench 17″ and the trapezoidal trench 17. It has beenrecognized that, in the Schottky barrier diode in which the p type layer23 is formed on the edge part 24 and the bottom wall 20 of thetrapezoidal trench 17, the electric field strength of the bottom wall 20of the trapezoidal trench 17 is weakened, and the part in which theelectric field strength reaches the maximum is shifted to the side wall22 of the trapezoidal trench 17 as shown in FIG. 8. More specifically,the electric field strength of the bottom wall 20 of the trapezoidaltrench 17 was weakened to 3×10⁵ V/cm or less, and the electric fieldstrength of the lower part of the side wall 22 of the trapezoidal trench17 was 1.5×10⁶ V/cm showing the maximum.

In the Schottky barrier diode of FIG. 9 that has the same arrangement asthat of FIGS. 1A, 1B, and 2, it has been recognized that the electricfield strength of the side wall 22 of the trapezoidal trench 17 isweakened by the p type layer 23 also formed on the side wall 22 of thetrapezoidal trench 17, and the part on which an electric field isconcentrated is placed away from the inner wall of the trapezoidaltrench 17. More specifically, the electric field strength of the sidewall 22 of the trapezoidal trench 17 was weakened to 3×10⁵ V/cm or less,and an area having an electric field strength of 1.5×10⁶ V/cm was absentaround the inner wall of the trapezoidal trench 17.

Thereafter, a relationship between a threshold voltage V_(th) and areverse leakage current J_(r) flowing when a voltage of 600 V is appliedwas examined by use of a Schottky barrier diode (see FIG. 2) having atrench structure, a Schottky barrier diode (see FIG. 10) having a JBS(Junction Barrier Schottky) structure, a Schottky barrier diode (seeFIG. 11) having a pseudo-JBS structure, and a Schottky barrier diode(see FIG. 12) having a planar structure.

The Schottky barrier diode of FIG. 10 (JBS structure) was produced asfollows.

First, an n⁻ type SiC epitaxial layer (concentration=1×10¹⁶ cm⁻³,thickness T=5 μm) was allowed to grow on an n⁺ type SiC substrate(concentration=1×10¹⁹ cm⁻³, thickness=250 μm, chip size=1.75 mm□), andthen aluminum (Al) ions were implanted in a multi-stage manner from thesurface of the SiC epitaxial layer toward the inside through a hard mask(SiO₂) that was subjected to patterning into a predetermined shape atimplanting energy=360 keV, dose amount=2.0×10¹² cm⁻², implantingenergy=260 keV, dose amount=1.5×10¹³ cm⁻², implanting energy=160 keV,dose amount=1.0×10¹³ cm⁻², implanting energy=60 keV, doseamount=2.0×10¹⁵ cm⁻², implanting energy=30 keV, and dose amount=1.0×10¹⁵cm⁻². Thereafter, the SiC epitaxial layer underwent heat-treatment(annealing treatment) for three minutes at 1775° C. As a result, a guardring and a JBS structure made of p type SiC were simultaneously formedon a surface layer part of the SiC epitaxial layer. Thereafter, a fieldinsulating film (SiO₂ thickness=15000 Å) was formed on the surface ofthe SiC epitaxial layer, and was subjected to patterning so that anactive region having a predetermined size was exposed, and then an anodeelectrode (Mo) was formed. After forming the anode electrode, a cathodeelectrode was formed on the reverse surface of the SiC substrate.

The Schottky barrier diode (pseudo-JBS structure) of FIG. 11 has ahigh-resistance pseudo-JBS structure (B implantation layer) in which theactivation rate of boron ions is less than 5%. The Schottky barrierdiode (pseudo-JBS structure) is produced by using boron (B) as animpurity instead of Al, and performing annealing treatment such atemperature (less than 1500° C.) as not to activate implanted impurityions while defect caused by a collision of implanted impurity ions inthe crystal structure of a wide bandgap semiconductor are recovered(crystallinity recovery).

The Schottky barrier diode (planar) of FIG. 12 can be produced throughthe same step as the Schottky barrier diode of FIG. 11 except that astep of forming a pseudo-JBS structure is not performed.

A relationship among the threshold voltage V_(th), the reverse leakagecurrent J_(r), the on-resistance R_(on)·A, and the breakdown voltageV_(B) of each Schottky barrier diode is shown in FIG. 13 to FIG. 15. Aspecific value of each characteristic is shown in the following table 1.

TABLE 1 Threshold Leakage current Breakdown voltage On-resistancedensity voltage V_(th)(V) R_(on) · A(mΩ · cm²) Jr(A/cm²) V_(BR)(V) Chipsize Active size JBS 0.930 1.23 4.48 × 10⁻⁶ 822 1.1 × 1.38 mm 1.116 mm²× 2 JBS 0.976 1.68 8.73 × 10⁻⁶ 971 1.87 mm□ 2.657 mm² JBS 0.637 1.284.22 × 10⁻³ 801 1.75 mm□ 2.28 mm² JBS 0.642 1.20 4.04 × 10⁻³ 805 1.75mm□ 2.28 mm² Planar 0.909 1.22 5.33 × 10⁻⁶ 970 1.84 mm□ 2.28 mm² Planar1.012 0.98 4.16 × 10⁻⁶ 951 1.75 mm□ 2.28 mm² Planar 0.947 1.12 1.85 ×10⁻⁵ 956 1.75 mm□ 2.28 mm² Planar 0.965 1.04 5.77 × 10⁻⁶ 950 1.75 mm□2.28 mm² Planar 0.977 0.99 6.32 × 10⁻⁶ 948 1.75 mm□ 2.28 mm² Planar0.987 0.93 5.54 × 10⁻⁶ 951 1.75 mm□ 2.28 mm² Planar 0.901 1.00 6.87 ×10⁻⁵ 961 1.75 mm□ 2.28 mm² Planar 0.900 1.01 4.13 × 10⁻⁵ 956 1.75 mm□2.28 mm² Planar 0.913 0.89 6.84 × 10⁻⁵ 946 1.75 mm□ 2.28 mm² Planar0.813 0.99 3.91 × 10⁻⁴ 930 1.75 mm□ 2.28 mm² Planar 0.776 1.01 8.76 ×10⁻⁴ 890 1.75 mm□ 2.28 mm² Planar 0.769 0.83 9.56 × 10⁻⁴ 888 1.75 mm□2.28 mm² Planar 0.587 0.95 3.46 × 10⁻¹ 608 1.75 mm□ 2.28 mm² Planar0.698 0.93 1.02 × 10⁻² 798 1.75 mm□ 2.28 mm² Pseudo-JBS 0.776 0.96 1.30× 10⁻⁴ 891 1.75 mm□ 2.28 mm² Pseudo-JBS 0.792 0.85 6.93 × 10⁻⁵ 923 1.75mm□ 2.28 mm² Pseudo-JBS 0.779 0.96 1.30 × 10⁻⁴ 926.7 1.75 mm□ 2.28 mm²Pseudo-JBS 0.875 0.99 4.35 × 10⁻⁵ 931.1 1.75 mm□ 2.28 mm² Pseudo-JBS0.859 1.06 4.69 × 10⁻⁵ 929.2 1.75 mm□ 2.28 mm² Pseudo-JBS 0.887 1.043.93 × 10⁻⁵ 928.6 1.75 mm□ 2.28 mm² Pseudo-JBS 0.894 0.89 3.81 × 10⁻⁵922.7 1.75 mm□ 2.28 mm² Trench 0.629 1.38 3.30 × 10⁻⁵ 870 1.75 mm□ 2.28mm² Trench 0.634 1.21 4.06 × 10⁻⁶ 741 1.75 mm□ 2.28 mm²

From FIG. 13 to FIG. 15 and Table 1, it has been recognized that theleakage current J_(r) has a tendency to rise when the threshold voltageV_(th) is lowered if the on-resistance R_(on)·A is at the same level inthe Schottky barrier diodes having a JBS structure, a planar structure,and a pseudo-JBS structure, whereas the leakage current J_(r) is kept ata small value even if the threshold voltage V_(th) is lowered in theSchottky barrier diode having the trench structure of the presentembodiment.

From these results, it has been recognized that a reverse leakagecurrent J_(r) of the whole of the Schottky barrier diode 1 can bereliably reduced in the Schottky barrier diode 1 shown in FIGS. 1A, 1B,and 2. In other words, in the Schottky barrier diode 1 having thestructure of FIGS. 1A, 1B, and 2, a reverse leakage current J_(r) can bereliably reduced even if a reverse voltage closer to a breakdown voltageV_(B) is applied, and therefore the pressure resistance of a widebandgap semiconductor can be satisfactorily utilized.

As a result, the threshold voltage V_(th) can be set at 0.3 V to 0.7 V,and the leakage current J_(r) in the normal rated voltage V_(R) can beset at 1×10⁻⁹ A/cm² to 1×10⁻⁴ A/cm², and therefore a current-carryingloss can be reduced to be equal to or to be smaller than that of anSi-pn diode while a switching loss can be smaller than the Si-pn diode.As a result, it is built in a power module for use in, for example, aninverter circuit that forms a driving circuit to drive an electric motorused as a power source for electric vehicles (including hybridautomobiles), trains, industrial robots, etc., and hence it is possibleto achieve a power module that is high in withstanding pressure and thatis low in loss.

Moreover, there is a possibility that the side wall 22 of thetrapezoidal trench 17 will be damaged during etching, and a Schottkybarrier cannot be formed between the side wall 22 and the anodeelectrode 27 according to predetermined design when the trapezoidaltrench 17 is formed by dry etching as at a step of FIG. 20C describedlater. Therefore, in the Schottky barrier diode 1 of the presentembodiment, the surface 12 of the SiC epitaxial layer 6 that is coveredwith a hard mask 35 (described later) and that is protected (step ofFIG. 20B described later) is used chiefly as a Schottky interface duringetching, and a p type layer 23 is formed on the damaged side wall 22. Asa result, the side wall 22 of the trapezoidal trench 17 can be usedeffectively. Additionally, a pn junction having a high barrier is formedat a part in the side wall 22 of the trapezoidal trench 17 that has ahigh electric field strength, and hence the leakage current J_(r) can bereduced.

<Effect of Built-in SiC-Pn Diode>

Next, referring to FIG. 16, a description will be given of an effectwhen a contact portion 26 is formed at the p type layer 23 and when a pndiode 25 is built in the SiC epitaxial layer 6.

FIG. 16 is a graph showing a current-voltage (I-V) curve of a built-inpn junction portion.

A current-carrying test was made by applying a forward voltage to theSchottky barrier diode having the structure of FIGS. 1A, 1B, and 2 whilevarying the forward voltage from 1 V to 7 V. Additionally, the amount ofvariation of an electric current flowing to the pn junction portion ofthe Schottky barrier diode when the applied voltage is varied from 1 Vto 7 V was evaluated.

On the other hand, the same current-carrying test as above was made withrespect to a Schottky barrier diode having the same structure as that ofFIGS. 1A, 1B, and 2 except that the contact portion 26 of the p typelayer 23 is not formed, and the amount of variation of an electriccurrent flowing to the pn junction portion was evaluated.

As shown in FIG. 16, in the pn junction portion in which the contactportion 26 is not formed at the p type layer 23, the electric currentwas substantially constant almost without being increased approximatelyfrom a point at which the applied voltage exceeds 4 V.

On the other hand, in the Schottky barrier diode in which the contactportion 26 is formed at the p type layer 23 and that has the built-in pndiode 25, the increasing rate of an electric current from a point atwhich the applied voltage exceeds 4 V rose more rapidly than theincreasing rate to 4 V or less.

As a result, it has been recognized that, in FIGS. 1A, 1B, and 2, if theanode electrode 27 (Schottky electrode) is kept in ohmic contact withthe pn diode 25 disposed in parallel in the Schottky barrier diode 1,part of the surge current can be allowed to flow to the built-in pndiode 25 by turning the built-in pn diode 25 on even if a large surgecurrent flows to the Schottky barrier diode. As a result, it has beenrecognized that the surge current flowing to the Schottky barrier diode1 can be reduced, and therefore the Schottky barrier diode 1 can beprevented from being thermally broken down by the surge current.

<Two Schottky Electrodes (First Electrode and Second Electrode)>

Next, referring to FIG. 17 and FIG. 18, a description will be given ofefficiency improvement of a reduction in the reverse leakage currentJ_(r) and in the threshold voltage V_(th) by being provided with twoSchottky electrodes (first electrode 28 and second electrode 29).

FIG. 17 is an enlarged view of a main part of the distribution view ofthe electric field strength shown in FIG. 9, in which a part near thetrench of the Schottky barrier diode is enlarged. FIG. 18 is a graphshowing the electric field strength distribution in a surface of a unitcell of the Schottky barrier diode shown in FIG. 17.

As described above, in the Schottky barrier diode 1 of the presentembodiment, the electric field strength of the unit cell 18 in thesurface 12 can be weakened by forming the trapezoidal trench 17 and byforming the p type layer 23 on the bottom wall 20 and the side wall 22of the trapezoidal trench 17. Therefore, there is a case in which a parthaving a relatively high electric field strength and a part having arelatively low electric field strength are present like a relationshipbetween the central part 31 and the peripheral edge 30 of the unit cell18 although the electric field strength distributed on the surface 12 ofthe unit cell 18 does not cause an increase in the reverse leakagecurrent J_(r) as an absolute value.

More specifically, as shown in FIG. 17 and FIG. 18, an electric fieldstrength of 0 MV/cm to 8.0×10⁵ MV/cm is distributed on the peripheraledge 30 of the unit cell 18 serving as a first part of the semiconductorlayer, and an electric field strength of 8.0×10⁵ MV/cm to 9.0×10⁵ MV/cmis distributed on the central part 31 of the unit cell 18 serving as asecond part of the semiconductor layer. In the electric field strengthdistribution shown when a reverse voltage is applied, the electric fieldstrength (second electric field) of the central part 31 of the unit cell18 is higher than the electric field strength (first electric field) ofthe peripheral edge 30 of the unit cell 18.

Therefore, for example, a p type polysilicon that forms a comparativelyhigh potential barrier (e.g., 1.4 eV) is allowed to make a Schottkyjunction, which serves as the first electrode 28, with the central part31 of the unit cell 18 to which a relatively high electric field isapplied. If the electrode is a semiconductor electrode made of, forexample, polysilicon, there is a possibility that semiconductors thatdiffer from each other in bandgap will be connected together accordingto heterojunction instead of Schottky junction.

On the other hand, for example, aluminum (Al) that forms a comparativelylow potential barrier (e.g., 0.7 eV) is allowed to make a Schottkyjunction, which serves as the second electrode 29, with the peripheraledge 30 of the unit cell 18 to which a relatively low electric field isapplied.

As a result, in the central part 31 of the unit cell 18 to which arelatively high electric field is applied when a reverse voltage isapplied, a reverse leakage current J_(r) can be restrained by a highSchottky barrier between the first electrode 28 (polysilicon) and theSiC epitaxial layer 6 (second Schottky barrier).

On the other hand, in the peripheral edge 30 of the unit cell 18 towhich a relatively low electric field is applied, even if the height ofa Schottky barrier between the second electrode 29 (aluminum) and theSiC epitaxial layer 6 is lowered, there is little fear that a reverseleakage current J_(r) will flow beyond this Schottky barrier. Therefore,when the Schottky barrier (first Schottky barrier) is made low, anelectric current can be allowed to preferentially flow at a low voltagewhen a forward voltage is applied.

Therefore, it has been recognized that the reverse leakage current J_(r)and the threshold voltage V_(th) can be efficiently reduced by properlyselecting the anode electrode 27 (Schottky electrode) in accordance withthe distribution of the electric field strength of the unit cell 18 whena reverse voltage is applied.

<Impurity Concentration of SiC Epitaxial Layer>

Next, referring to FIG. 19, a description will be given of the magnitudeof impurity concentration of the SiC substrate 2 and the magnitude ofimpurity concentration of the SiC epitaxial layer 6.

FIG. 19 is a view to describe the impurity concentration of the SiCsubstrate and the impurity concentration of the SiC epitaxial layer.

As shown in FIG. 19, each of the SiC substrate 2 and the SiC epitaxiallayer 6 is made of an n type SiC that contains n type impurities. Themagnitude relationship among impurity concentrations of these componentsis expressed as SiC substrate 2>buffer layer 7>layers 8 to 10.

The concentration of the SiC substrate 2 is constant, for example, at5×10¹⁸ to 5×10¹⁹ cm⁻³ along its thickness direction. The concentrationof the buffer layer 7 is constant, for example, at 1×10¹⁷ to 5×10¹⁸ cm⁻³along its thickness direction, or is low along its surface.

The concentrations of the drift layers 8 to 10 vary in a step-by-stepmanner with each interface of the base drift layer 8, the low-resistancedrift layer 9, and the obverse-surface drift layer 10 as a boundary. Inother words, there is a concentration difference between the layer onthe surface side (12) and the layer on the reverse surface side (11)with respect to each interface.

The concentration of the base drift layer 8 is constant, for example, at5×10¹⁴ to 5×10¹⁶ cm⁻³ along its thickness direction. The concentrationof the base drift layer 8 may be continuously lowered from about 3×10¹⁶cm⁻³ to about 5×10¹⁵ cm⁻³ in proportion to an approach to the surfacefrom the reverse surface 11 of the SiC epitaxial layer 6 as shown by thebroken line of FIG. 19.

The concentration of the low-resistance drift layer 9 is higher than theconcentration of the base drift layer 8, and is constant, for example,at 5×10¹⁵ to 5×10¹⁷ cm⁻³ along its thickness direction. Theconcentration of the low-resistance drift layer 9 may be continuouslylowered from about 3×10¹⁷ cm⁻³ to about 5×10¹⁵ cm⁻³ in proportion to anapproach to the surface from the reverse surface 11 of the SiC epitaxiallayer 6 as shown by the broken line of FIG. 19.

The concentration of the obverse-surface drift layer 10 is lower thanthe concentration of the base drift layer 8 and the concentration of thelow-resistance drift layer 9, and is constant, for example, at 5×10¹⁴ to1×10¹⁶ cm⁻³ along its thickness direction.

As shown in FIGS. 1A, 1B, and 2, in the unit cell 18 (line cell)partitioned by the stripe-like trapezoidal trench 17, an area (currentpath) in which an electric current can be allowed to flow is restrictedby the width of the pitch P of the trapezoidal trench 17, and thereforethere is a fear that the resistance value of the unit cell 18 will riseif the impurity concentration of a part that forms the unit cell 18 inthe SiC epitaxial layer 6 is low.

Therefore, as shown in FIG. 19, the concentration of the low-resistancedrift layer 9 forming the base part of the unit cell 18 is made higherthan the concentration of the base drift layer 8, and, as a result, theresistance value of the unit cell 18 can be restrained from rising bythe low-resistance drift layer 9 that has a comparatively highconcentration even if the current path is restricted by the pitch P ofthe trapezoidal trench 17. As a result, the unit cell 18 can be made lowin resistance.

On the other hand, the electric field strength to be applied to thesurface 12 of the SiC epitaxial layer 6 when a reverse voltage isapplied can be reduced by providing the obverse-surface drift layer 10that has a comparatively low concentration on the surface layer part ofthe unit cell 18 contiguous to the anode electrode 27 (Schottkyelectrode). As a result, the reverse leakage current J_(r) can be madeeven smaller.

<Method for Forming Trench and P Type Layer>

Next, referring to FIG. 20A to FIG. 20D, a description will be given ofa method for forming the trapezoidal trench 17 shown in FIG. 2, which isemployed as one example, and the p type layer 23.

FIG. 20A to FIG. 20D are views showing a method for forming the trenchshown in FIG. 2 and the p type layer in order of steps.

First, as shown in FIG. 20A, the buffer layer 7, the base drift layer 8,the low-resistance drift layer 9, and the obverse-surface drift layer 10are subjected to epitaxial growth on the SiC substrate 2 in this order.

Thereafter, as shown in FIG. 20B, a hard mask 35 made of SiO₂ is formedon the surface 12 of the SiC epitaxial layer 6 according to, forexample, a CVD (Chemical Vapor Deposition) method. Preferably, thethickness of the hard mask 35 is 1 μm to 3 μm. Thereafter, the hard mask35 is subjected to patterning by a well-known photolithography techniqueand an etching technique. At this time, etching conditions are set sothat the amount (thickness) of etching is 1 to 1.5 times as thick as thethickness of the hard mask 35. More specifically, if the thickness ofthe hard mask 35 is 1 μm to 3 μm, etching conditions (gas kind, etchingtemperature) are set so that the amount of etching is 1 μm to 4.5 μm. Asa result, the amount of over-etching with respect to the SiC epitaxiallayer 6 can be made smaller than a general amount, and therefore an edgepart 37 inclined at angle θ₁ (100° to 170°>90°) with respect to thesurface 12 of the SiC epitaxial layer 6 can be formed at the lower partof the side wall of the opening 36 of the hard mask 35 that hasundergone etching.

Thereafter, as shown in FIG. 20C, the SiC epitaxial layer 6 is subjectedto dry etching from the surface 12 to a depth in which its deepest partreaches a halfway part of the low-resistance drift layer 9 through thehard mask 35, and, as a result, the stripe-like trapezoidal trench 17 isformed. The etching conditions at this time are set at gas kind:O₂+SF₆+HBr, Bias: 20 W to 100 W, and Internal pressure of the device: 1Pa to 10 Pa. As a result, the edge part 24 of the bottom wall 20 can beshaped to be curved. Additionally, the edge part 37 having apredetermined angle θ₁ is formed at the lower part of the side wall ofthe opening 36 of the hard mask 35, and therefore the side surface 21 ofthe trapezoidal trench 17 can be inclined at angle θ₁ with respect tothe bottom surface 19 of the trapezoidal trench 17.

Thereafter, as shown in FIG. 20D, a p type impurity (e.g., aluminum(Al)) is implanted toward the trapezoidal trench 17 through the hardmask 35 while leaving the hard mask 35 used to form the trapezoidaltrench 17. The doping of the p type impurity is achieved by an ionimplantation method in which, for example, the implanting energy is 380keV, and the dose amount is 2×10¹³ cm⁻². After performing the doping ofthe impurity, the p type layer 23 is formed by performing annealingtreatment at, for example, 1775° C.

According to this forming method, ion implantation is performed by useof the hard mask 35 used when the trapezoidal trench 17 is formed, andtherefore a step for forming a mask is not required to be added when thep type layer 23 is formed.

Additionally, the trapezoidal trench 17 according to predetermineddesign can be accurately formed by appropriately adjusting the thicknessof the hard mask 35, and impurities can be prevented from beingimplanted to parts (e.g., top of the unit cell 18) other than thetrapezoidal trench 17 during ion implantation. Therefore, an n typeregion for Schottky junction with the anode electrode 27 can be secured.

Still additionally, in the trapezoidal trench 17, not only the bottomwall 20 but also all of the side wall 22 is allowed to face the open endof the trapezoidal trench 17. Therefore, when a p type impurity isimplanted to the SiC epitaxial layer 6 through the trapezoidal trench17, the impurity that has entered the inside of the trapezoidal trench17 from the open end of the trapezoidal trench 17 can be allowed toreliably impinge on the side wall 22 of the trapezoidal trench 17. As aresult, the p type layer 23 can be formed easily.

<Relationship Between Trench and SiC Crystal Structure>

Next, a relationship between a trench and an SiC crystal structure willbe described with reference to FIG. 21.

FIG. 21 is a schematic view that represents a unit cell having a 4H-SiCcrystal structure.

Various kinds of SiC compounds that differ in crystal structure fromeach other, such as 3C—SiC, 4H—SiC, and 6H—SiC, can be mentioned as SiCfor use in the Schottky barrier diode 1 of the present embodiment.

Among these SiC compounds, the crystal structure of 4H-SiC can beapproximated by a hexagonal system, and is formed such that four carbonatoms are combined with one silicon atom. The four carbon atoms arepositioned at four vertexes of a regular tetrahedron in which thesilicon atom is disposed at the center. In these four carbon atoms, onesilicon atom is positioned in the direction of the [0001] axis withrespect to the carbon atom, and the other three carbon atoms arepositioned on the [000-1] axis side with respect to the siliconatomic-group atom.

The [0001] axis and the [000-1] axis are in the axial direction of ahexagonal column, and a surface (top surface of the hexagonal column)whose normal is the [0001] axis is a (0001) plane (Si plane). On theother hand, a surface (lower surface of the hexagonal column) whosenormal is the [000-1] axis is a (000-1) plane (C plane).

Each side surface of the hexagonal column whose normal is the [1-100]axis is a (1-100) plane, and a surface that passes through a pair ofridge lines not adjoining each other and whose normal is the [11-20]axis is a (11-20) plane. These are crystal planes perpendicular to the(0001) plane and perpendicular to the (000-1) plane.

Preferably, in the present embodiment, the SiC substrate 2 whoseprincipal surface is the (0001) plane is used, and the SiC epitaxiallayer 6 is grown so that the (0001) plane becomes a principal surfacethereon. Additionally, preferably, the trapezoidal trench 17 is formedso that the plane orientation of the side surface 21 becomes a (11-20)plane.

<Modifications of Cross-Sectional Shape of Trench>

Next, modifications of the cross-sectional shape of the trapezoidaltrench 17 will be described with reference to FIG. 22A to FIG. 22F.

FIGS. 22A to 22F are views showing modifications of the cross-sectionalshape of the trench, and FIG. 22A is a first modification, FIG. 22B is asecond modification, FIG. 22C is a third modification, FIG. 22D is afourth modification, FIG. 22E is a fifth modification, and FIG. 22F is asixth modification.

In the trapezoidal trench 17, as shown in, for example, FIG. 22A, thecontact portion 26 may be formed over the entire inner surface of thetrapezoidal trench 17 from the bottom wall 20 to the opening end of thetrapezoidal trench 17 through the edge part 24 in the same way as the ptype layer 23.

Although only a case in which the cross-sectional shape of thetrapezoidal trench 17 is formed such that the side surface 21 of eachtrapezoidal trench 17 is inclined at angle θ₁ (>90°) with respect to thebottom surface 19 has been mentioned as an example in the descriptionwith reference to FIG. 2 and FIG. 3, the cross-sectional shape of thetrench is not limited to this.

For example, the trapezoidal trench is not required to incline the wholeof the side surface 21, and a part of the side surface 39 (lower part 42of the side surface 39) may be selectively trapezoidal (tapered), forexample, as in a selective trapezoidal trench 41 of FIG. 22B or FIG.22C, and other parts of the side surface 39 (upper part 43 of the sidesurface 39) may make an angle of 90° with the bottom surface 19. In thiscase, the p type layer 23 is formed only at the lower part 42(trapezoidal part) of the side surface 39 through the edge part 24 fromthe bottom wall 20 of the selective trapezoidal trench 41. Additionally,the contact portion 26 may be formed only at the bottom wall 20 of theselective trapezoidal trench 41 as shown in FIG. 22B, or may be formedto the upper end of the lower part 42 of the side surface 39 from thebottom wall 20 of the selective trapezoidal trench 41 through the edgepart 24 in the same way as the p type layer 23 as shown in FIG. 22C.

Likewise, in the structure of FIG. 22B or FIG. 22C, the lower part 42 ofthe side surface 39 faces the open end of the selective trapezoidaltrench 41, and therefore the p type layer 23 can be formed easily.

The selective trapezoidal trench 41 of FIG. 22B can be formed, forexample, through steps shown in FIG. 23A to FIG. 23D.

More specifically, first, as shown in FIG. 23A, the buffer layer 7, thebase drift layer 8, the low-resistance drift layer 9, and theobverse-surface drift layer 10 are subjected to epitaxial growth on theSiC substrate 2 in this order.

Thereafter, as shown in FIG. 23B, a hard mask 38 made of SiO₂ is formedon the surface 12 of the SiC epitaxial layer 6 according to, forexample, the CVD method. Preferably, the thickness of the hard mask 38is 1 μm to 3 μm. Thereafter, the hard mask 38 is subjected to patterningby a well-known photolithography technique and an etching technique. Atthis time, etching conditions are set so that the amount (thickness) ofetching is 1.5 to 2 times as thick as the thickness of the hard mask 38.More specifically, if the thickness of the hard mask 38 is 1 μm to 3 μm,etching conditions (gas kind, etching temperature) are set so that theamount of etching is 1.5 μm to 6 μm. These etching conditions areconditions for setting the amount of over-etching greater than theamount of over-etching set when the hard mask 35 is etched according tothe step of FIG. 20B. As a result, an edge part 44 that is inclined atangle θ₁ (91° to 100°>90°) with respect to the surface 12 of the SiCepitaxial layer 6 and that is smaller than the edge part 37 (see FIG.20B) can be formed at the lower part of the side wall of the opening 40of the hard mask 38 that has undergone etching.

Thereafter, as shown in FIG. 23C, the SiC epitaxial layer 6 is subjectedto dry etching from the surface 12 to a depth in which its deepest partreaches a halfway part of the low-resistance drift layer 9 through thehard mask 38, and, as a result, the stripe-like selective trapezoidaltrench 41 is formed. The etching conditions at this time are set at gaskind: O₂+SF₆+HBr, Bias: 20 W to 100 W, and Internal pressure of thedevice: 1 Pa to 10 Pa. As a result, the edge part 24 of the bottom wall20 can be shaped to be curved. Additionally, the edge part 44 smallerthan the edge part 37 is formed at the lower part of the side wall ofthe opening 40 of the hard mask 38, and therefore only the lower part 42of the side surface 39 of the selective trapezoidal trench 41 can beinclined at angle θ₁ with respect to the bottom surface 19, and theupper part 43 of the side surface 39 can be made at 90° (perpendicular)with respect to the bottom surface 19.

Thereafter, as shown in FIG. 23D, a p type impurity (e.g., aluminum(Al)) is implanted toward the selective trapezoidal trench 41 throughthe hard mask 38 while leaving the hard mask 38 used to form theselective trapezoidal trench 41. The doping of the p type impurity isachieved by an ion implantation method in which, for example, theimplanting energy is 380 keV, and the dose amount is 2×10¹³ cm⁻². Afterperforming the doping of the impurity, the p type layer 23 is formed byperforming annealing treatment at, for example, 1775° C.

The trench is not required to incline the side surface 22, and, as in aU-shaped trench 45 of FIG. 22D, FIG. 22E, or FIG. 22F, the side surface21 may make an angle of 90° (perpendicular) with respect to the bottomsurface 19. In this case, the p type layer 23 may be formed from thebottom wall 20 of the U-shaped trench 45 to the opening end of theU-shaped trench 45 through the edge part 24 as shown in FIG. 22D andFIG. 22E, or may be formed only at the bottom wall 20 and the edge part24 of the U-shaped trench 45 as shown in FIG. 22F. Additionally, thecontact portion 26 may be formed only at the bottom wall 20 of theU-shaped trench 45 as shown in FIG. 22D and FIG. 22F, or may be formedfrom the bottom wall 20 of the U-shaped trench 45 to the opening end ofthe U-shaped trench 45 through the edge part 24 as shown in FIG. 22E inthe same way as the p type layer 23.

The U-shaped trench 45 of FIG. 22D can be formed, for example, throughsteps shown in FIG. 24A to FIG. 24G.

First, as shown in FIG. 24A, the buffer layer 7, the base drift layer 8,the low-resistance drift layer 9, and the obverse-surface drift layer 10are subjected to epitaxial growth on the SiC substrate 2 in this order.

Thereafter, as shown in FIG. 24B, a hard mask 46 made of SiO₂ is formedon the surface 12 of the SiC epitaxial layer 6 according to, forexample, the CVD (Chemical Vapor Deposition) method. Preferably, thethickness of the hard mask 46 is 1 μm to 3 μm. Thereafter, the hard mask46 is subjected to patterning by a well-known photolithography techniqueand an etching technique. At this time, etching conditions are set sothat the amount (thickness) of etching is 2 to 3 times as thick as thethickness of the hard mask 46. More specifically, if the thickness ofthe hard mask 46 is 1 μm to 3 μm, etching conditions (gas kind, etchingtemperature) are set so that the amount of etching is 2 μm to 6 μm.These etching conditions are conditions for setting the amount ofover-etching greater than the amount of over-etching set when the hardmask 38 is etched according to the step of FIG. 23B. As a result, thelower part of the side wall of the opening 47 of the hard mask 46 thathas undergone etching can be formed at an angle of 90° (perpendicular)with respect to the surface 12 of the SiC epitaxial layer 6.

Thereafter, as shown in FIG. 24C, a p type impurity (e.g., aluminum(Al)) is implanted toward the surface of the SiC epitaxial layer 6through the hard mask 46 that has undergone patterning. The doping ofthe p type impurity is achieved by an ion implantation method in which,for example, the implanting energy is 380 keV, and the dose amount is2×10¹³ cm⁻². After performing the doping of the impurity, the p typelayer 48 is formed by performing annealing treatment at, for example,1775° C.

Thereafter, as shown in FIG. 24D, the SiC epitaxial layer 6 is subjectedto dry etching from the surface 12 to a depth penetrating the bottompart of the p type layer 48 through the hard mask 46 while leaving thehard mask 46 used to form the p type layer 48, and, as a result, astripe-like intermediate trench 53 is formed. As a result, the remainder(lateral part) of the p type layer 48 remains at the side wall of theintermediate trench 53.

Thereafter, as shown in FIG. 24E, a p type impurity (e.g., aluminum(Al)) is implanted toward the intermediate trench 53 through the hardmask 46 while leaving the hard mask 46 used to form the intermediatetrench 53. The doping of the p type impurity is achieved by an ionimplantation method in which, for example, the implanting energy is 380keV, and the dose amount is 2×10¹³ cm⁻². After performing the doping ofthe impurity, annealing treatment is performed at, for example, 1775°C., and, as a result, the implanted impurity mixes with the impurity ofthe p type layer 48, and a p type layer 54 is formed.

Thereafter, as shown in FIG. 24F, the SiC epitaxial layer 6 is subjectedto dry etching from the surface 12 to a depth penetrating the bottompart of the p type layer 54 through the hard mask 46 while leaving thehard mask 46 used to form the p type layer 54, and, as a result, thestripe-like U-shaped trench 45 is formed. As a result, the remainder(lateral part) of the p type layer 54 remains at the side wall 22 of theU-shaped trench 45.

Thereafter, as shown in FIG. 24G, a p type impurity (e.g., aluminum(Al)) is implanted toward the U-shaped trench 45 through the hard mask46 while leaving the hard mask 46 used to form the U-shaped trench 45.The doping of the p type impurity is achieved by an ion implantationmethod in which, for example, the implanting energy is 380 keV, and thedose amount is 2×10¹³ cm⁻². After performing the doping of the impurity,annealing treatment is performed at, for example, 1775° C., and, as aresult, the implanted impurity mixes with the impurity of the p typelayer 54, and the p type layer 23 is formed.

As described above, even if the side surface 21 of the U-shaped trench45 is perpendicular to the bottom surface 19, the p type layer 23 can bereliably formed at the side wall 22 of the U-shaped trench 45 byrepeatedly performing a step of forming the p type layers 48 and 54 eachof which has a predetermined depth from the surface 12 by performing ionimplantation toward the surface 12 of the SiC epitaxial layer 6 and astep of forming the trenches 53 and 45 penetrating the bottom parts ofthe p type layers 48 and 54 and of leaving the lateral parts of the ptype layers 48 and 54 at the side walls of the trenches 53 and 45. Therepetition of the ion implantation and the trench formation is notlimited to two times, and may be three, four, or more times.

Additionally, ion implantation is performed while continuously using thehard mask 46 that has been used when the p type layers 48, 54 and thetrenches 53, 45 are formed, and therefore there is no need to add a stepof forming a mask when the p type layer 23 is formed.

Although the embodiment of the present invention has been described asabove, the present invention can be embodied in other modes.

For example, although a variation of a Schottky barrier diode in which atrench is formed in the SiC epitaxial layer 6 has been shown as oneexample of the present invention in the aforementioned embodiment, thepresent invention is not limited to this variation in which a trench isformed, and no specific limitations are imposed on the shape of asemiconductor device if it is a semiconductor device whose thresholdvoltage V_(th) is 0.3 V to 0.7 V and whose leakage current J_(r) in therated voltage V_(R) is 1×10⁻⁹ A/cm² to 1×10⁻⁴ A/cm². For example, it maybe the aforementioned JBS structure, the aforementioned planarstructure, and the aforementioned pseudo-JBS structure.

Additionally, an arrangement may be employed in which the conductivitytype of each semiconductor part of the Schottky barrier diode 1 isinverted. For example, in the Schottky barrier diode 1, the part of a ptype may be an n type, and the part of an n type may be a p type.

Additionally, the epitaxial layer is not limited to an epitaxial layermade of SiC, and it may be a wide bandgap semiconductor other than SiC,such as a semiconductor having an insulation breakdown electric fieldgreater than 2 MV/cm, and, more specifically, it may be GaN (whoseinsulation breakdown electric field is about 3 MV/cm and whose bandgapwidth is about 3.42 eV), or may be diamond (whose insulation breakdownelectric field is about 8 MV/cm and whose bandgap width is about 5.47eV).

Additionally, the planar shape of the trench is not required to be likestripes, and it may be, for example, a lattice trench 55 shown in FIG.25. In this case, a unit cell 56 is formed in a rectangularparallelepiped shape at each window part of the lattice trench 55.Additionally, preferably, the lattice trench 55 is formed so that theplane orientation of a side surface becomes a (11-20) plane and a(1-100) plane.

Additionally, an insulating film may be formed on a part of or all ofthe inner surface (bottom surface and side surface) of a trench. Forexample, in FIG. 26 to FIG. 30, each of the insulating films 57 to 61 isformed on a part of or all of the side surface 21 and the bottom surface19 of the trapezoidal trench 17.

More specifically, the insulating film 57 of FIG. 26 is embedded fromthe bottom surface 19 of the trapezoidal trench 17 to the opening end ofthe trapezoidal trench 17 so that its upper surface becomes flush withthe surface 12 of the SiC epitaxial layer 6, and is contiguous to theentire surface of both of the bottom surface 19 and the side surface 21.

The insulating film 58 of FIG. 27 is embedded from the bottom surface 19of the trapezoidal trench 17 to an intermediate part in the depthdirection of the trapezoidal trench 17, and is contiguous to the entiresurface of the bottom surface 19 and to a part of the side surface 21.

The insulating film 59 of FIG. 28 is formed into a thin film reachingthe opening end of the trapezoidal trench 17 through the edge part 24from the bottom wall 20 so as to leave a space in the trapezoidal trench17. As a result, it is contiguous to the entire surface of both of thebottom surface 19 and the side surface 21 of the trapezoidal trench 17.

The insulating film 60 of FIG. 29 is formed into a thin film with whichthe peripheral edge 30 of the opening end of the trapezoidal trench 17is covered from the surface side (12) through the edge part 24 from thebottom wall 20 so as to leave a space in the trapezoidal trench 17. As aresult, it is contiguous to the entire surface of both of the bottomsurface 19 and the side surface 21 of the trapezoidal trench 17.

The insulating film 61 of FIG. 30 is formed into a thin film reaching anintermediate part in the depth direction of the trapezoidal trench 17 inthe side surface 21 through the edge part 24 from the bottom wall 20 soas to leave a space in the trapezoidal trench 17. As a result, it iscontiguous to the entire surface of the bottom surface 19 of thetrapezoidal trench 17 and to a part of the side surface 21.

The capacity can be reduced by forming each of the insulating films 57to 61 at a part of or all of the side surface 21 and the bottom surface19 of the trapezoidal trench 17 in this way, and therefore the switchingspeed can be increased.

Additionally, in the example of FIG. 31, a part of the n typeobverse-surface drift layer 10 is replaced with a p type surface layer10′ that has been made into a p type one, and the anode electrode 27 isbrought into contact with this p type surface layer 10′, and, as aresult, it is possible to provide a pn diode 62 composed of the p typesurface layer 10′ and the n type SiC epitaxial layer 6 (low-resistancedrift layer 9). Therefore, it is possible to obtain the same effect asin the pn diode 25 of FIG. 16. Additionally, in the example of FIG. 32,the p type layer 23 is formed only to the intermediate part in the depthdirection of the trapezoidal trench 17, and the p type layer 23 iscovered and hidden with the insulating film 58. In this case, in thesame way as in FIG. 31, a pn diode 62 can be provided by replacing apart of the n type obverse-surface drift layer 10 with a p type surfacelayer 10′ that has been made into a p type one and by bringing the anodeelectrode 27 into contact with this p type surface layer 10′.

Additionally, a Schottky junction (heterojunction) can be made with theSiC epitaxial layer 6 by use of, for example, molybdenum (Mo) ortitanium (Ti) as an anode electrode besides, for example, aluminum andpolysilicon mentioned above.

Additionally, for example, Al (aluminum) can be used as a p typeimpurity to form the p type layer 23.

Additionally, the p type layer 23 is not necessarily required to beformed.

The semiconductor device (semiconductor power device) of the presentinvention can be built in a power module for use in, for example, aninverter circuit that forms a driving circuit to drive an electric motorused as a power source for electric vehicles (including hybridautomobiles), trains, industrial robots, etc. Additionally, it can bebuilt in a power module for use in an inverter circuit that convertspower generated by a solar battery, a wind generator, or other powergenerators (particularly, a private electric generator) so as to matchthe electric power of a commercial power source.

The embodiments of the present invention are merely specific examplesused to clarify the technical contents of the present invention, and thepresent invention should not be understood as being limited to theseexamples, and the scope of the present invention is to be determinedsolely by the appended claims.

Additionally, the components shown in each embodiment of the presentinvention can be combined together within the scope of the presentinvention.

The present application corresponds to Japanese Patent Application No.2011-165660 filed in the Japan Patent Office on Jul. 28, 2011, and theentire disclosure of the application is incorporated herein byreference.

REFERENCE SIGNS LIST

-   -   1 Schottky barrier diode    -   2 SiC substrate    -   6 SiC epitaxial layer    -   7 Buffer layer    -   8 Base drift layer    -   9 Low-resistance drift layer    -   10 Obverse-surface drift layer    -   11 Reverse surface (of SiC epitaxial layer)    -   12 Surface (of SiC epitaxial layer)    -   17 Trapezoidal trench    -   18 Unit cell    -   19 Bottom surface (of trench)    -   20 Bottom wall (of trench)    -   21 Side surface (of trench)    -   22 Side wall (of trench)    -   23 P type layer    -   24 Edge part    -   25 Pn diode    -   26 Contact portion    -   27 Anode electrode    -   28 First electrode    -   29 Second electrode    -   30 Peripheral edge (of unit cell)    -   31 Central part (of unit cell)    -   41 Selective trapezoidal trench    -   42 Lower part of side surface (of selective trapezoidal trench)    -   43 Upper part of side surface (of selective trapezoidal trench)    -   45 U-shaped trench    -   55 Lattice trench    -   56 Unit cell

1. A semiconductor device, comprising: a substrate; a first conductivetype epitaxial layer, formed on a surface of the substrate; a fieldinsulating film, covering a portion of a surface of the first conductivetype epitaxial layer, wherein the portion of the surface of the firstconductive type epitaxial layer comprises a field region and an activeregion surrounded by the field region; and an electric field moderatingpart, positioned in a portion of the field region closer to the activeregion, wherein a top surface of the electric field moderating part islower than a surface of the active region, and wherein the electricfield moderating part comprises a second conductive type layer.
 2. Thesemiconductor device according to claim 1, further comprising: aplurality of trench in the active region, each trench having a sidewalland a bottom; and a Schottky electrode, contacting at least a portion ofthe surface of the active region.
 3. The semiconductor device accordingto claim 2, the bottom and the sidewall of the trench in the activeregion further comprising the electric field moderating part.
 4. Thesemiconductor device according to claim 3, wherein the Schottkyelectrode is formed to fill the trenches; and the electric fieldmoderating part has a contact portion at the bottom wall of the trench,and an ohmic junction is formed between the contact portion and theSchottky electrode filling the trenches.
 5. The semiconductor deviceaccording to claim 3, wherein a part of the first conductive typeepitaxial layer that is different from the electric field moderatingpart has a first conductive type first part for applying a firstelectric field and a first conductive type second part for applying asecond electric field that is stronger than the first electric fieldwhen a reverse voltage is applied; and the Schottky electrode comprises:a first electrode, for forming a first Schottky barrier between thefirst electrode itself and the first part, and a second electrode, forforming a second Schottky barrier higher than the first Schottky barrierbetween the second electrode itself and the second part.
 6. Thesemiconductor device according to claim 3, wherein a part of the firstconductive type epitaxial layer that is different from the electricfield moderating part has a first conductive type first part forapplying a first electric field and a first conductive type second partfor applying a second electric field that is stronger than the firstelectric field when a reverse voltage is applied; the Schottky electrodecomprises: a first electrode, for forming a first Schottky barrierbetween the first electrode itself and the first part, and a secondelectrode, for forming a second Schottky barrier higher than the firstSchottky barrier between the second electrode itself and the secondpart; and the first part of the first conductive type epitaxial layer isformed on peripheral portions of the opening ends of the trenches of asurface layer portion of the first conductive type epitaxial layer; andthe second part of the first conductive type epitaxial layer is formedon a part of the surface layer portion of the first conductive typeepitaxial layer and adjacent to the peripheral portions.
 7. Thesemiconductor device according to claim 2, wherein the electric fieldmoderating part is disposed on a bottom of a ring-shaped trenchsurrounding the active region, and wherein a depth of the trench in theactive region and a depth of the ring-shaped trench are substantiallythe same.
 8. The semiconductor device according to claim 2, the trenchcomprises a tapered trench, and the tapered trench has a planar bottomwall and side walls each having at least a portion inclining at an anglegreater than 90° relative to the planar bottom wall.
 9. Thesemiconductor device according to claim 2, wherein the trenches comprisestrip-shaped strip trenches.
 10. The semiconductor device according toclaim 2, wherein the trenches comprise lattice-shaped lattice trenches.11. The semiconductor device according to claim 1, wherein the electricfield moderating part is further positioned at a bottom of a ring-shapedtrench surrounding the active region.
 12. The semiconductor deviceaccording to claim 1, wherein the first conductive type epitaxial layercomprises: a base drift layer having a first impurity concentration; anda low-resistance drift layer formed on the base drift layer having asecond impurity concentration that is higher than the first impurityconcentration; and the trench is formed with a deepest portion of thetrench reaching the low-resistance drift layer and is separated by aunit cell being a part of the first conductive type epitaxial layer. 13.The semiconductor device according to claim 12, wherein the firstimpurity concentration of the base drift layer decreases along adirection from a back surface of the first conductive type epitaxiallayer to a surface.
 14. The semiconductor device according to claim 12,wherein the second impurity concentration of the low-resistance driftlayer is fixed along a direction from a back surface of the firstconductive type epitaxial layer to a surface.
 15. The semiconductordevice according to claim 12, wherein the second impurity concentrationof the low-resistance drift layer decreases along a direction from aback surface of the first conductive type epitaxial layer to a surface.16. The semiconductor device according to claim 12, wherein the firstconductive type epitaxial layer further comprises a surface drift layerformed on the low-resistance drift layer, and the surface drift layerhas a third impurity concentration that is lower than the secondimpurity concentration.
 17. The semiconductor device according to claim12, wherein the substrate comprises a wide bandgap semiconductor; andthe first conductive type epitaxial layer further comprises a bufferlayer formed on the substrate, and the buffer layer has a fourthimpurity concentration that is higher than the first impurityconcentration.
 18. The semiconductor device according to claim 1,wherein the first conductive type epitaxial layer comprises a widebandgap semiconductor.
 19. The semiconductor device according to claim18, wherein an insulation breakdown electric field of the wide bandgapsemiconductor is greater than 1 MV/cm.
 20. The semiconductor deviceaccording to claim 18, wherein the wide bandgap semiconductor is SiC,GaN, AlN or diamond.